Photocatalysis technology including hydrogen evolution from water splitting, CO2 reduction and N2 conversion to ammonia emerges as a significant approach for energy crisis and environmental pollution. For these conventional semiconductors such as TiO2, ZnO, WO3, CdS and g-C3N4, however, inefficient photoabsorption, rapid recombination of photogenerated carriers, and inadequate surface reactive sites hamper the photoinduced activity and stability. Defect engineering, especially oxygen vacancy, has recently drawn the attention of a number of investigators primarily in connection with its feasibility of regulatability, identifiability and effectiveness. A series of ferroelectric and piezoelectric semiconductors, with internal electric field generated by the polarization, are considered an excellent candidate for replacement of conventional semiconductors, because the observed charge separation ability of those is far from theoretical expectation. With the boost of oxygen vacancy, polarization behavior can be effectively regulated to further improve photocatalytic performance. Related studies based on the above background are the current hotspot of photocatalysis; this paper reviews the latest research progress of ferroelectric and piezoelectric photocatalysts with oxygen vacancy. Starting from the generation of oxygen vacancies, five preparation strategy including ion doping, thermal treatment, chemical reduction, ultraviolet irradiation, and plasma etching are introduced; advanced characterization are summarized in classification of spectroscopy, energy spectrum, electron microscopy, density function theory and in situ techniques. Secondly, the mechanism of oxygen vacancy regulated polarization and their synergistic photocatalytic reactions are reviewed and summarized. Finally, an overview on the prospect of advanced photocatalytic engineering concerned to oxygen vacancies involved ferroelectric and piezoelectric photocatalysts is proposed.

Advances in ferroelectric and piezoelectric photocatalysts with oxygen vacancy
    Abstract
        Graphical abstract
    1 Introduction
    2 The formation of OVs
    3 The characterization of OVs
        3.1 Spectroscopy
        3.2 Energy spectrum
        3.3 Electron microscope
        3.4 In situ techniques
        3.5 Density function theory (DFT)
    4 The photocatalytic materials involving OVs
        4.1 OV-cooperated ferroelectric materials
        4.2 OV-cooperated piezoelectric materials
    5 Summary and prospect
    Acknowledgements 
    References

• Shuang Zhao(School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China)
• Lu Han(School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China)
• Bochao Tian(School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China)
• Zhongyu Duan(School of Materials and Metallurgy, University of Science and Technology Liaoning, Anshan 114051, China)
• Ruidan Su(Department of Computer Science and Engineering, Shanghai Jiao Tong University, Shanghai, China)
• Xibao Li(School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China)
• Yiyang Wan(School of Materials Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China)